Yeah, that's the bg problem with Heroes, the crap writing. Good guys turn to bad guys, bad guys to good guys, then they're back again in two episodes for no good reason. And when Sylar started talking about "OMG THE HUNGER" it was the lowest point in the show, so thank fuck that put a stop to that shit. But then they seem to be having a redemption moment with Sylar that was going somewhat well, and suddenly he's on the beach in a romantic moment with his girlfriend and BAM kills her, bad guy again, no reason.
And yeah, the shapeshifting thing is stupid. Not only has it been done before, but it's been done before ON THIS SHOW. But I still keep watching because I've become invested in the story and have to see how it turns out.